To use a polyolefin resin for specific applications, the polyolefin resin should have superior toughness, strength, environmental stress and crack resistance etc. Such characteristics can be easily improved by increasing molecular weight of the polyolefin resin, however, this disadvantageously deteriorates the moldability of the polyolefin resin. Because of the disadvantages of such polyolefin resins, it is preferable to use a polyolefin resin having a single physical property alone while adjusting the structure of the polymer or using a suitable processing aid, rather than using polyolefin resins having different physical properties. However, generally the polyethylene resin prepared with Ziegler-Natta and metallocene catalyst has narrow molecular weight distribution, thus if used alone, various problems may occur. Thus, using a polymer having a broad molecular weight distribution or a multimodal molecular weight distribution, the moldability of the polyolefin resin can be improved with maintaining characteristics of toughness, strength, environmental stress, crack resistance and melt strength etc., thereby solving the disadvantage of the polyolefin resin having narrow molecular weight distribution.
The polyolefin having multimodal molecular weight distribution is a polyolefin containing at least two components each having different molecular weight, and for example, includes a high molecular weight component and a low molecular weight component in relatively proper proportions. Many studies have been conducted for the preparation of a polyolefin having broad molecular weight distribution or multimodal molecular weight distribution. One method among them is a post-reactor process or a melting blending process in which polyolefins having at least two different molecular weights are blended before or during the processing of the polyolefin. For example, U.S. Pat. No. 4,461,873 discloses a blending method of physically bending two different kinds of polymers for preparing a bimodal polymer blend. When such a physical blending method is used, it is liable to produce a molded form having high gel component, a product appearance is deteriorated owing to the gel component, and thus the polyolefin cannot be used for the films. Further, the physical blending method requires a complete uniformity, so there is a disadvantage of the preparing cost being increased.
Another method for preparing polyolefin having multimodal molecular weight distribution, for example bimodal molecular weight distribution is to use a multistage reactor. In the multistage reactor which includes two or more reactors, a first polymer component having one molecular weight distribution among two different molecular weight distribution of the bimodal polymer, is prepared in a certain condition at a first reactor, the first polymer component prepared is transferred to a second reactor, and then a second polymer component having different molecular weight distribution from that of the first polymer component, is prepared in a different condition from that of the first reactor, at the second reactor. The above-mentioned method solves the problems relating to the gel component, but it uses the multistage reactor, so the production efficiency may be decreased or the production cost may be increased. Also, when the high molecular weight components are prepared in the first reactor, the low molecular weight components are not prepared in the second reactor and thus the finally manufactured polyolefin particles may be made only by the high molecular weight components.
Still another method for preparing polyolefin having broad molecular weight distribution or multimodal molecular weight distribution is to polymerize the polyolefin by using a mixture of catalysts in a single reactor. Recently, in the pertinent art, the various attempts have been made for producing polyolefin having broad molecular weight distribution or multimodal molecular weight distribution, by using two or more different catalysts in a single reactor. In this method, the resin particles are uniformly mixed in a level of sub-particles, thus the resin components each having different molecular weight distribution exists in a single phase. For example, U.S. Pat. Nos. 4,530,914 and 4,935,474 disclose a method for producing a polyolefin having a broad molecular weight distribution by polymerizing ethylene or higher alpha-olefins in the presence of a catalyst system comprising two or more metallocenes and aluminoxanes having different reaction development and termination rate constants. Also, U.S. Pat. Nos. 6,841,631 and 6,894,128 disclose a method for preparing polyethylene having bimodal or multimodal molecular weight distribution by using a metallocene-type catalyst comprising at least two metal compounds and the usage of the polyethylene for manufacturing films, pipes, hollow molded articles and so on. Polyethylene produced in this way has a good processability, but the dispersed state of the polyethylene component per the molecular weight in unit particle is not uniform, so there are disadvantages of rough appearance and unstable physical properties even in relatively good processing conditions.
U.S. Pat. No. 4,937,299 discloses a method for preparing polyolefin by using a catalyst system comprising at least two kinds of metallocenes each having different reactivity ratio with respect to monomer to be polymerized. U.S. Pat. No. 4,808,561 discloses a method for preparing olefin polymerization supported catalyst by reacting metallocene with alumoxane in the presence of a carrier. The metallocene is supported in the carrier to form solid power catalyst. As the carrier, inorganic oxide materials such as silica, alumina, silica-alumina, magnesia, titania, zirconia and the mixture thereof, and resinous materials such as polyolefin (for example, finely divided polyethylene) can be employed, and the metallocenes and alumoxanes are deposited on the dehydrated carrier material.
Generally, since the polymer prepared with Ziegler-Natta catalyst alone or metallocene catalyst system has a narrow molecular weight distribution, it is not made to prepare the satisfactory polyolefin which has a multimodal molecular weight distribution or broad molecular weight distribution. Accordingly, in the related art, a method has been known for preparing a bimodal resin by using a mixture catalyst system containing Ziegler-Natta catalyst and metallocene catalyst components. The mixture catalyst system typically includes a combination of heterogeneous Ziegler-Natta catalysts and homogeneous metallocene catalyst. The mixture catalyst system is used for preparing the polyolefin having a broad molecular weight distribution or bimodal molecular weight distribution, to adjust the molecular weight distribution and polydispersity of the polyolefin.
U.S. Pat. No. 5,539,076 discloses a mixture catalyst system of metallocene/non-metallocene for preparing a specific bimodal high-density copolymer. The catalyst system is supported by an inorganic carrier. The mixture catalyst of Ziegler-Natta and metallocene supported has relatively low activity than single uniform catalyst, so it is difficult to prepare polyolefin having properties suitable for a specific use. In addition, since polyolefin is prepared in a single reactor, the gel which is generated in the blending method may be produced, it is difficult to insert comonomer to high molecular weight components part, the form of polymer produced may be poor and further two polymer components may not be uniformly mixed, so the quality control of the produced polyolefin may be difficult.
Korean Patent No. 1437509 discloses a catalyst composition and a polymerization method for producing multimodal polyolefin resins having a low extrusion load and much extrusion amount during the molding to have excellent productivity. The resin is a mixture of two or more different kinds of polymers, so it exhibits excellent processability despite a high molecular weight and a low melt flow index. However, this resin has a rough surface when machining large diameter pipes. Journal of POLYMER ENGINEERING AND SCIENCE, JULY 2004, Vol. 44, No. 7 1283-1294 discloses that in the extrusion molding, when the molten resin undergoes high stress, there is a difference in elastic energy according to the position inside the extruder, and the polymer chains move to be energy equilibrium. In bimodal or multimodal polyolefin, the difference in the molecular weight of each polymer chain is larger than that of general monodomain polyolefin, therefore, the elastic energy difference is high, and the movement of the polymer chains is easier. For this reason, it is difficult to evenly distribute the molecules, which may cause problems such as surface roughness.
In addition, conventional bimodal products have a great difference in elastic energy between polymer chains, and when the polymer undergoes high stress at the time of processing, the polymer chains having relatively low molecular weight and low elastic energy move toward the wall surface of the extruder having high stress and polymer chains with high molecular weight and high elastic energy move into the center having relatively low stress. As a result, the polymer chains having low molecular weight are distributed in the outer surface of a molded product (for example, a pipe), and problems of surface roughness and melt fracture (phenomenon that the surface is not smooth and ruggedly broken during polymer processing) are issued.